This invention relates to a diagnostic apparatus which utilises terahertz radiation. Embodiments of the invention may be useful for a range of diagnostic methods providing spectroscopic and tomographic information. The specific applications of the apparatus may be diverse and include, again not exhaustively, medical imaging, chemical analyses and non-destructive testing.
Terahertz (T-ray or THz) radiation lies on the boundary of electronics (millimeter waves) and photonics (infrared). The terahertz spectrum encompasses the wavelenths in the range of 3 mm to 15 xcexcm although it will be appreciated that these limits are indicative rather than absolute.
Terahertz radiation exhibits a large range of modifications on passage through varying materials or on reflection from materials. Such changes include attenuation or partial attenuation of different frequencies of the waveform and other alteration of the waveform depending upon the material through which the radiation or pulses pass. Terahertz radiation interacts strongly with polar molecules, a prime example being water. Water molecules absorb terahertz waves, on the one hand limiting penetration of the radiation in moist substances, and on the other hand making it readily detectable even in very low concentrations. It can be used for detecting low concentrations of polar gases. However, terahertz radiation will penetrate non-polar substances such as fats, cardboard, cloth and plastics with little attenuation. Materials including organic materials have varying transmission, reflection and absorption characteristics to terahertz radiation.
Accordingly, use of terahertz radiation can indicate the presence of different materials. Terahertz radiation has been used for an increasing range of chemical sensing applications, including biomedical diagnostics (Han et al. (2000), Optics letters 25(4) 242-244) semiconductor device diagnostics (Walecki et a., (1993) Applied Physics letters 63(13) 1809-1811), trace gas analysis (Jacobsen Optics letters 21(24) 2011-2013) moisture analysis for agriculture (Hadjiloucas et al., (1999) IEEE Transactions on Microwave Theory and Techniques 47(2) 142-149) quality control of packed goods (May (1997) New Scientist 154 (2083) 22) inspection of artwork and inspection of internal structure of smart cards (Nuss (1996) IEEE Circuits and Devices 12(2) 25-30). Having low average power, T rays are particularly attractive for medical applications, where it is important to avoid damaging the sample.
Terhertz chemical sensing has in recent years been applied to a number of biological problems. As indicated above T rays are strongly attenuated by moist tissue because of water absorption. This has limited medical applications to dry or thin samples. Toshiba, for example, have explored T-ray images of human teeth (Arnone et al (1999) xe2x80x9cApplications of terahertz (THz) technology to medical imaging,xe2x80x9d in Proceedings of SPIExe2x80x94Conference on Terahertz Spectroscopy and Applications vol 3828 209-219 SPIE (Munich Germany)). The T-ray data revealed differences between the enamel, the enamel and dentine and a cavity. T-ray images of living plant leaves and thin samples of wood have been studied to show wear and density profiles (Koch xe2x80x9cTHz imaging: Fundamentals and biological applicationsxe2x80x9d in Proceedings of SPIExe2x80x94Conference on Terahertz Spectroscopy and Applications vol 3828 202-208 SPIE (Munich Germany). Rice University has shown terahertz profiles of burnt chicken tissue (Mittleman et al (1999) Applied Physics B Lasers ad Optics 68(6) 1085-1094) and thin slices of Spanish ham have also been studied (Ferguson and Abbott (2000) xe2x80x9cSignal processing for t-ray bio-sensor systemsxe2x80x9d in Proceedings of SPIE""s 2000 Symposium on Smart materials and MEMS, SPIE (Melbourne, Australia)). The problems with biological imaging are resolution, penetration and speed. The resolution is limited by wavelength in the far field, giving about 0.3 mm resolution at 1 terahertz, which will be sufficient for many biological applications. Depth penetration is a greater problem, even for reflective spectroscopy. Depth penetration can be improved by increasing the terahertz power and reducing the path length. Lastly, the imaging speed is important for living samples that tend to move. A CCD two dimensional imaging technique has been used to minimise motion between the imager and the sample.
Both transmissive and reflective geometries have been used or at least proposed in terahertz devices. Transmission geometries include placing a sample between the transmitter and the detector of the terahertz radiation. This often requires that the terahertz radiation follows a long path length. Where the path is through an atmosphere containing water vapour there is poor terahertz radiation transmission and so detection is made more difficult if at all possible. Similarly reflective geometries have suffered from the utilisation of long path lengths. To alleviate this problem it has been proposed in transmission geometries to place the transmitter, sample and detector within a container in which the atmosphere permits ready transmission of terahertz radiation. This restricts the application of terahertz to samples that can be fitted within the container and are not adversely affected by the atmosphere within the container, and which therefore generally excludes medical application.
A further problem is that terahertz radiation is typically low powered and even a few millimeters of moist dermal tissue can effectively block transmission.
It is a proposed object of this invention to provide a diagnostic apparatus to obviate or minimise at least one of the aforementioned problems, or at least provide the public with a useful choice.
The invention may be said to reside, not necessarily in the broadest or only form, in a diagnostic apparatus including a terahertz generator for generating terahertz radiation, and an enclosure including a reflection receiving window and a terahertz detector for detecting terahertz radiation, the terahertz generator directing terahertz radiation onto a target, reflected terahertz radiation being reflected through the reflection receiving window into the enclosure and to the detector, a modified atmosphere being provided within the enclosure to permit ready transmission of terahertz radiation. The terahertz radiation may be generated inside or on a surface of the enclosure, and is directed out from the reflection receiving window. Preferably the terahertz generator is radiated by terahertz inducing laser radiation directed thereonto by a laser, the terahertz generator being either a terahertz generating electro-optic crystal or a terahertz generating photoconductive dipole antennna. The terahertz generator may be a terahertz generating electro-optic crystal, and preferably the terahertz generating crystal forms at least part of the reflection receiving window. A beam splitter may be positioned between the laser and the terahertz generator to split off a probe laser beam from the terahertz inducing laser radiation, said probe laser beam travels through a probe laser path to the detector means, the detector providing a quantitative output, the output being determined by the amplitude of that part of the reflected teraherz radiation wave coaligned with the probe laser at the detector. Preferably a delay positioned in the probe laser path said delay altering a length of the probe laser path over terahertz subwavelength distances to thereby vary the co-alignment of the probe beam and the reflected terahertz radiation wave.